1. Field of the Invention
The present invention relates to a projection type lens meter for measuring refractive power or astigmatic axis of spectacle lenses, and more particularly to an optical system adapted for use in such a projection type lens meter.
2. Description of the Prior Art
A projection lens meter requires a projection magnification of at least 30 to 40 times in order to achieve a satisfactory accuracy of measurement, so that there is usually employed a projection lens of focal length in a range of 800 to 1,000 mm. Because of such a long focal length the optical path from the projection lens to the screen constituting the image plane, that is, the back focus becomes inevitably long, requiring a large image space. Using a conventional projection type lens meter it is known to reduce the image space by folding the optical path with four mirrors. There are however certain limitations on the positions of the projection lens and of the screen in order to ensure satisfactory operational performance, and it is therefore difficult to reduce the dimension of the image space and to reduce the size of the projection type lens meter. Although the optical system itself may be accommodated within a smaller space by the use of a larger number of mirrors with corresponding increased number of reflections, it becomes impossible to obtain a projected image of a desired dimension due to vignetting of the light beam, and the manufacture of such optical system is extremely difficult as the adjustment of each reflecting surface becomes more complicated and has to be more accurate.